This invention relates generally to fluid flow measuring devices. More particularly, this invention relates to a thermal based fluid flow measuring instrument utilizing either a fixed ratio parameter or a constant difference arrangement of thermoresistive sensors in determining the mass flow rate of a fluid.
Methods exist for determining the flow rate for fluids, including gases, flowing through a system, such as a pipe or conduit. Correspondingly, there are many disadvantages associated with these flow rate determination methods. Accordingly, the invention described herein discloses an apparatus for determining the flow rate of a fluid flowing through a system, such as a conduit or a pipe. Advantageously, the apparatus is operable to determine the fluid flow rate economically and without the disadvantages associated with conventional fluid flow rate methods. Additionally, the invention removes the temperature dependence of the thermoresistive sensors inherent in the constant xcex94T methods. This in turn enables the associated xcex94T to be reduced between the heated and reference thermoresistive sensors. In one preferred embodiment, the xcex94T is about 20xc2x0 C. or less which is preferable for operation in liquid fluids.
More particularly, the invention described herein is operable to separate sensor drift from fluid property (thermal conductivity, viscosity, Prandtl number, etc.) dependence, enabling an independent modeling of the fluid property dependence characteristic. This fluid property dependence is included in a complete and thorough modeling of the heat transfer process using a universal flow correlation and which also includes thermoresistive sensor internal thermal resistance and stem losses. The model is programmed in a microprocessor and calibration constants are obtained by calibration in a primary fluid such as water. The model is then used, along with measurements from the thermoresistive sensors, to calculate mass flow. Use in other fluids is facilitated by substitution of the new fluid thermophysical properties and stem loss characteristics, essentially eliminating recalibration for fluids with known thermophysical properties.
In accordance with the invention, an apparatus is provided for determining a mass flow rate of a fluid having temperatures ranging within an expected range. The apparatus includes a first sensor mounted in the fluid having a resistance that is substantially a linear function of temperature within the expected temperature ranges of the fluid. Also mounted in the fluid is a second sensor having a resistance that is substantially a linear function of temperature within the expected temperature ranges of the fluid. A first current source is connected to the first sensor and applies a current (IC) through and a voltage across the first sensor, producing a first resistance RC and a first sensor voltage EC. A second current source is connected to the second sensor and applies a current (IH) through and a voltage across the second sensor, producing a second resistance RH and a second sensor voltage EH. A controller controls the first and second current sources by providing control signals to the first and second current sources that provides a level of current to the heated sensor sufficient to keep the heated to reference sensor resistance ratio constant under all conditions of flow. Proper operation dictates that the ratio of the heated to reference sensors remain fixed so the controller varies the reference sensor current such that the IH/IC ratio remains constant. Preferably, the controller produces an output corresponding to the mass flow rate of the fluid, based on the control signals sent to the first and second current sources. This output signal along with a signal representative of the ambient fluid temperature are sent to a microprocessor for precise determination of the fluid mass flow rate. It is preferred that the apparatus is first calibrated prior to measuring mass flow rate of a fluid and the calibration information is stored in the microprocessor memory for utilization in the determination of the mass flow rate. The microprocessor is also configured to calibrate the apparatus in various fluids.
Preferably, the microprocessor includes memory and has various operational modes, including modes of calibration and mass flow rate determination. During the calibration mode, the first and second sensors are located in a fluid having known thermophysical properties. As the fluid is flowing, the controller outputs a signal representative of the current, IH, provided to the second sensor. A fluid temperature signal is also output and the temperature of the calibration fluid may be varied over a range of temperatures corresponding to expected operating temperatures of the apparatus. During calibration, it is also necessary to independently measure the mass flow rate of the fluid. The mass flow rate is measured by the independent instrument over a range of values from about zero to the maximum desired. These three sets of measurements (temperature, mass flow rate and current) are used by the microprocessor to regress an equation derived from thermodynamic principles, including a universal flow correlation, which relates the heated sensor current to the mass flow and which includes the temperature dependent thermophysical properties of the fluid in which the calibration is being conducted. Also included in the equation are expressions for the internal thermal resistance of the heated thermoresistive sensors and for the fraction of power input to the heated sensor which is lost into the stem of the probe. Accordingly, IH the heated sensor current versus mass flow rate is regressed. The fluid temperature is used to obtain appropriate values of various fluid thermophysical properties at each IH and mass flow rate using an equation or lookup table. The regression produces a set of constants used in the equation relating the mass flow rate and the heated current which minimizes the percent mass flow rate difference between measured and calculated values of mass flow rate for the specific calibration. These constants are stored in memory along with the fluid thermophysical properties versus temperature and the heated sensor stem loss profile versus mass flow.
During the measurement mode, the IH versus mass flow rate equation is reformulated for mass flow rate in terms of heated sensor current, IH. Thereafter, for each set of representative IH and fluid temperature inputs obtained, the equation relating mass flow rate to IH is solved by the microprocessor in real time, using stored stem loss data and thermophysical properties for the measured temperatures. Furthermore, since the general equation relating mass flow rate to IH is valid for the sensors calibrated in any fluid, all that is required to measure mass flow rate in an alternate fluid is to substitute the probe stem loss versus mass flow and thermophysical property versus temperature data of the new fluid for that of the calibrated fluid. Although in practice this procedure is currently limited to related classes of fluids such as aqueous, light hydrocarbon, heavy hydrocarbon, etc., the method essentially eliminates recalibration when measuring the mass flow rate of fluids having known thermophysical properties.
In an alternative embodiment of the invention, a device is provided for determining the mass flow rate of a fluid. The device includes a first sensor mounted in the fluid having a resistance which is a substantially linear function of a fluid temperature. The device also uses a second sensor having a resistance which is a substantially linear function of the fluid temperature mounted in the fluid at a location relative to the first sensor. A first current source is connected to the first sensor and applies a fixed current (IC) through and a voltage across the first sensor, producing a first sensor resistance RC and a first sensor voltage VC. A second current source is connected to the second sensor and applies a current (IH) through and a voltage across the second sensor, producing a second sensor resistance RH and a second sensor voltage VH. A first fixed resistor produces a signal corresponding to an amount of current (IC) through the first sensor and a second fixed resistor produces a signal corresponding to an amount of current (IH) through the second sensor. The device also includes an analog-to-digital converter for converting the first sensor voltage VC, second sensor voltage VH, first fixed resistor signal and second fixed resistor signal to digitized values, producing digitized output signals therefor. A microprocessor calculates the resistances of the first and second sensors based on the voltages and representative currents. The microprocessor calculates the value of the heated sensor resistance, RH required to maintain a constant ratio between the first and second sensor resistances (RH/RC). Based on the difference between measured and required RH values, the microprocessor directs the heated sensor current, IH, to be increased or decreased. The digital-to-analog converter provides the appropriate control voltage to the heated sensor current source. According to this embodiment, the reference sensor current source remains constant. The entire process is repeated periodically in a manner that provides the appropriate current, under any fluid flow conditions, that maintains the RH/RC ratio constant. A fluid ambient temperature is also calculated from the reference sensor resistance. Accordingly, the mass flow rate of the fluid is proportional to the amount of current (IH) required to maintain the ratio RH/RC constant. The microprocessor is operable to calibrate the device and to produce an output representing the mass flow rate of the fluid.
In yet another embodiment of the invention, an apparatus is provided for determining the mass flow rate of a fluid having known thermophysical properties. The apparatus includes a sensing means having a resistance which is a function of temperature. A current source is connected to the sensing means and applies a current through and a voltage across the sensing means. A fixed resistor is located in series with the sensing means and the current source, for outputting a signal indicative of the current through the sensing means which is used to calculate resistance of the sensing means. The apparatus also includes a reference sensor operable to provide a signal indicative of the fluid temperature. A processor means determines the mass flow rate of the fluid utilizing the fluid thermophysical properties and calibration information based on a universal flow correlation. The apparatus maintains a constant difference between a temperature of the sensing means and the fluid temperature during calibration and the same difference is maintained during operation of the apparatus to determine the mass flow rate of the fluid. The processor means outputs a signal indicative of the mass flow rate of the fluid based on the calibration information and current provided by the current source to maintain the constant temperature difference.